This invention relates to uninterruptible power supply (UPS) systems which provide electric power to critical loads when primary power supplies fail due to total power losses or deterioration which is that the power does not satisfy the end users' requirements.
UPS systems are widely used to assure that, when a primary power supply fails due to equipment malfunction, downed lines or other reasons, electric power will continue to be supplied to critical loads such as hospital operating room equipment, computer systems and computerized manufacturing equipment. UPS systems avoid equipment failures, costly downtime and equipment damage.
UPS systems traditionally take two basic forms, inverter-based and rotary. A typical inverter-based UPS system has a utility-powered rectifier connected to a DC buss which charges a string of chemical storage batteries. When the primary power supply fails, electronic circuitry converts direct current from the batteries into alternating current which operates the critical load. In an off-line or line-interactive UPS, this AC output is used to power the critical load only when power is unavailable from the primary power supply. In a double conversion UPS, the AC output provides power to the critical load at all times.
A typical rotary UPS uses a motor which drives a generator. The generator supplies alternating current to the critical load at all times. The motor is typically a DC motor that is driven during normal operations by rectified DC from the primary power supply, and driven during primary power supply interruptions by a battery string. During very brief power interruptions, the rotational momentum of the motor and generator can supply power to the critical load.
For inverter-based and rotary UPS systems, flywheel systems are available as a clean and reliable alternative or complement to chemical batteries. Such flywheel systems include a flywheel connected to an electrical machine which can operate as a motor and as a generator. The electrical machine is powered by the DC buss to operate as a motor when acceptable power is being received from the primary power supply. When power from the primary power supply fails, the electrical machine is rotated by the kinetic energy of the flywheel, and it acts as a generator to supply power to the DC buss.
Large UPS systems in the range of 20KW to 2 MW often use prime movers (fuel-burning engines) to drive backup generators during prolonged power outages. The prime movers are costly and complicated, and they require extensive ongoing maintenance. The engines may fail to start, resulting in loss of power to the critical load. In some localities, ordinances limit the running time or the number of starts per year of the engines for backup generators, which limits testing and overall usefulness of such systems.
Energy storage systems currently used to provide power to a DC buss are expensive and complicated. In battery energy storage systems, there is a risk that undetected battery damage or corrosion of battery terminals will result in a failure to deliver power when needed. Batteries have a limited life and they require expensive ventilation, drainage, air conditioning and frequent maintenance. Flywheel energy storage systems, while avoiding most disadvantages of batteries, are expensive since they are mechanically complex and they require complicated power electronics.
In some existing systems, power from the primary power supply is rectified and transmitted to a DC buss, converted to low frequency AC by a converter, and used to power a critical load. The associated UPS system has a high speed gas turbine, and a backup generator driven by the turbine. The backup generator is a brushless permanent magnet alternator which generates high frequency AC which is rectified and transmitted to the DC buss. The DC output buss provides power to an inverter, and the inverter converts the DC to a low frequency AC which powers the critical load. When the power from the primary power supply is present, the turbine rotor is stationary. When a brief outage occurs, a battery string supports the DC buss. When there is an extended power failure, a battery is connected to the generator which then, acting as a motor, brings the turbine rotor up to speed. When a predetermined minimum speed is attained, fuel is supplied to the gas turbine to sustain the rotation of the turbine, and power from the generator is supplied to the DC buss. Such systems are expensive and complicated compared to the present invention because they require a separate energy storage system.
A primary object of the present invention is to provide a UPS which is less complicated and less expensive than existing battery/generator/turbine UPS systems of the type described above.